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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

20% of the world’s population will eventually have to migrate away from coasts swamped by rising oceans. Cities including New York, London, Rio de Janeiro, Cairo, Calcutta, Jakarta and Shanghai would all be submerged.

The authors looked at past climate change events and model simulations of the future. They found a clear, strong relationship between the total amount of carbon pollution humans emit, and how far global sea levels will rise. The issue is that ice sheets melt quite slowly, but because carbon dioxide stays in the atmosphere for a long time, the eventual melting and associated sea level rise are effectively locked in.

As a result, the study authors found that due to the carbon pollution humans have emitted so far, we’ve committed the planet to an eventual sea level rise of 1.7 meters (5.5 feet). If we manage to stay within the 1 trillion ton carbon budget, which we hope will keep the planet below 2°C warming above pre-industrial levels, sea levels will nevertheless rise a total of about 9 meters (30 feet). If we continue on a fossil fuel-heavy path, we could trigger a staggering eventual 50 meters (165 feet) of sea level rise.

Right now we’re in a warm interglacial period, having come out of the last ice age (when New York City and Chicago were under an ice sheet) about 12,000 years ago. During that transition, the Earth’s average surface temperature warmed about 4°C, but that temperature rise occurred over a period of about 10,000 years.

In contrast, humans have caused nearly 1°C warming over the past 150 years, and we could trigger anywhere from another 1 to 4°C warming over the next 85 years, depending on how much more carbon we pump into the atmosphere.

Atmospheric carbon dioxide levels (top) and average global surface temperature (bottom) from the past 20,000 years to the next 10,000 years in various emissions scenarios. Illustration: Clark et al. (2016), Nature Climate Change.

What humans are in the process of doing to the climate makes the transition out of the last ice age look like a casual stroll through the park. We’re already warming the Earth about 20 times faster than during the ice age transition, and over the next century that rate could increase to 50 times faster or more. We’re in the process of destabilizing the global climate far more quickly than happens even in some of the most severe natural climate change events.

That rapid climate destabilization is what has climate scientists worried.

wili - In that Hansen and Sato 2011 they point out that we shouldn't ignore outlier estimates, noting that "...a 10-year doubling time was plausible, pointing out that such a doubling time from a 1mm per year ice sheet contribution to sea level in the decade 2005-2015 would lead to a cumulative 5 meter rise by 2095."

Note that they don't claim that is likely, but that it is plausible, and in the next paragraph note that "...more plausible but still accelerated conditions could lead to sea level rise of 80 cm by 2100." Note "more plausible" in that sentence. They then go on to discuss strong negative feedbacks as well.

The point they were making in that paper was that we can't at this time exclude extremely high sea level rise, and that more study is needed. They weren't claiming 5m by 2100 was the likely outcome, just that it wasn't impossible - despite multiple out of context claims by climate denialists, which seem IMO to be poor attempts to discredit Hansen.

The title says "Earth is warming 50x faster than when it comes out of an ice age," but the article says "We’re already warming the Earth about 20 times faster than during the ice age transition, and over the next century that rate could increase to 50 times faster or more."

Thanks for the clarification, KR. IRRC, Hansen somewhere also said we couldn't competely rule out a doubling time of 5 years. But I don't have the source at my fingertips, so maybe that's my memory playing tricks on me.

I still wonder why one shouldn't just multiply 50 (or 20, or whatever) to the rates given in #s 1 and 2 in my first post to get an approximation of the rates we should expect going forward. I know that there was a lot more ice then in the northern hemisphere, but as I understand it, the Antarctic ice sheet wasn't really in play then. And in any case, wouldn't a lower starting point just be another element to put into the same equation?

As I understand it, he's saying that Thwaites could destabilize at any time, and whenever it does, it is most likely to go in a matter of decades, leading to a rather abrupt sea level rise of about a 4 meters in the northern hemisphere. That is pretty shocking. Is that your understanding of the message?

(see GRACE data up through this year here, only one paper in disagreement, Zwally et al, and there are potential issues with that methodology), Greenland ice decreasing at 287 billion tons per year, Arctic ice is on a steep downward trend (albeit with yearly variations that do not reach statistical significance):

Why leave out the Arctic Sea ice which is also setting a new record every day for lowest sea ice ? Alan Motorman is using last year's denier meme, there is a reason the deniers are not using it this year.

Roughly yes. He did think there was still some uncertainty about whether Thwaites is committed to going, and he also thought that it was possibly a century or 2 before is really ramps up, but then it is a multi-decadal rather than multicentury process for the WAIS.

The description of including the fracture mechanics of high ice-cliffs and how that can drive retreat was fascinating/horrifying. And that including that into the ice models 'solved' the Plioscene was telling.

Like the best discoveries, once the original realisation that a mechanism needs to be included has been made, in hindsight it can seem like 'well, of course'. But in reality those are important advances in understanding.

It is probably one of those things a layman doesn't question when they see that land terminating glaciers taper down to their end while sea terminating glaciers end in ice cliffs. But when you think it through and realise the implications a lightbulb moment happens.

Sounds like this is new findings from including the fracture mechanics of ice cliffs.

Assentially it seems to be saying that an ice edge more than 1000 meters high, so 100 above the water, 900 below, in water 900 meters deep or more is unstable. So a sheet 200, 500, 800 meters thick at it's edge is stable, whether it is floating or not.

The ice in the WAIS is largely over 1000 meters thick then tapers to less than that at places like the Thwaites. So the edge of the Thwaites doesn't suffer from this instability. But if it retreats back into deeper water, where the ice is thicker, it can become unstable.

I am not sure what the last entered date is. Perhaps Gingerb has a better graph. 2015 goes way up, but 1997 did also. 2014 does not look bad to me. It looks like the data was close to the long term line until the start of 2015. We can hope that the data will return to the long term mean like it did in 1998.

He might be refering to this graph from AVISO Data. If you look only at the data from Jason-2 you get a SLR rate of 4.31 mm/yr. Of course this is a very short time span, so I do not thin it has any significance as of yet.

Thanks, ms and noa. Looking at earlier years, one might assume that this most recent divergence from the linear trend is just that--a temporary blip. But of course, it could be the beginning of a new, steeper linear, or the beginning of an exponential trend. If the latter, the question is what the doubling time will be.

As with many things, we won't know for sure till it's in the rearview mirror.

We are likely to be in a La Nina by year end and even if not, another La Nina will come and with it, a redistribution of water onto land. Just as surely will be deniers cries that "sealevel is falling". It makes no sense to be concerned about short term trends. With his "just saying", I am pretty sure Gingerbaker is aware of that too.

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